1. Field of the Invention
The present invention relates to a reflector having a high reflectivity of visible light, in particular for use in glass products, such as a mirror, etc., and it relates to a manufacturing method thereof.
2. Description of Prior Art
Generally, upon a mirror is formed a metal film having a high reflectivity, such as a film of silver, aluminum, etc. With such the mirror using silver, a solution of silver salt is applied upon a glass plate, to thereby form a silver layer or film through a reductive reaction on the surface of the glass plate. However, the silver film itself is easily oxidized, and also is easily eroded, therefore being very weak or inferior in the durability thereof. Accordingly, it is impossible to obtain a mirror by this method alone that is practically useful or endurable, and there is a necessity of forming a protection layer upon the surface of the silver film. Further, because of encroachment or invasion of water and so on from the edge (i.e., an outer periphery edge portion of the silver film), there is also a necessity of protection treatment even on the edge thereof, therefore a special protective coating treatment must be carried out, in particular, for the purpose of obtaining the mirror using silver which can be used on a washstand and/or in a bathroom. Also in many cases, an opaque material is applied as the protection layer or film, therefore the mirror can be formed only as a back-coated mirror, whereby the mirror obtained cannot be prevented from causing a dual-image due to the reflection of light upon a front surface and upon a reverse surface thereof.
With the mirror using aluminum film in the place thereof, in general, the aluminum film is formed by a method such as a vacuum evaporation method and/or a spattering method, necessitating the use of a vacuum apparatus or equipment as a facility thereof. Also, as it takes a period of time for forming the film by this method, there is a tendency that the cost for manufacturing thereof is increased due to a loss of efficiency in production or productivity. Further, the aluminum film formed through the above-mentioned method comes to be one which is low in durability, therefore a protection film is necessary for producing a practical mirror by this method as well.
As a method not requiring the use of vacuum apparatus or equipment, a method for manufacturing a mirror having the reflectivity being equal to or higher than 70%, by forming one-by-one a reflection layer and a reflection enforcement layer, using a normal pressure CVD (Chemical Vapor Deposition) method, is disclosed in Japanese Laid-Open Patent Hei 6-183787 (1994). In more detail, disclosed is a mirror in which a film having a high refractive index, such as a silicon film, is used as the reflection layer, on which is formed the reflection enforcement layer, including a layer having a low refractive index, such as a film of silicon oxide or the like, and a film of high refractive index, such as a silicon film, or a film of tin oxide, titanium oxide, etc., sequentially applied.
In Japanese Laid-Open Patent Hei 6-183787 (1994), the silicon film is mainly used as a reflection layer, and as a low refractive index layer for forming the reflection enforcement layer is used a layer of silicon oxide, while as a high refractive index layer is used the film of silicon, titanium oxide, or tin oxide, etc.
In a case where the silicon film is used as the uppermost film, there is a necessity that a further protection layer is formed because of the low durability of the silicon layer.
In a case where the film of titanium oxide or tin oxide is used, though no such problem in the durability thereof arises, there is a necessity that the film of silicon oxide as the low refractive index layer must be equal to or greater than 70 nm in thickness, so as to make a reflection color inconspicuous, under the condition that the reflectivity to visible light is equal or greater than 75%. However, a large amount of powder is generated when forming the film of silicon oxide through a pyrolytic method, and this may be taken into the film as a contaminating foreign body or may cause pinholes therein, thereby reducing a yield rate. Further, the powder is accumulated or piled up in a periphery of the apparatus and/or in exhaust pipes thereof, thus presenting a cause of reduction in the stability of the film-forming process.
For avoiding the drawbacks mentioned above, there can be considered a countermeasure wherein the supplied amount of the raw material is reduced so as to slow down the speed of forming films, however it is difficult to obtain the film thickness being equal to or greater than 70 nm, by this method.
Also, in Japanese Laid-Open Patent Hei 8-508708 (1996), there is proposed a mirror, wherein the low refractive film is made to be equal to or greater than 10 nm and be equal to or less than 40 nm in the thickness thereof, and the film of tin oxide is formed upon the surface thereof. However, in this case, the reflectivity to visible light is equal to or greater than 35% and is less than 70%, and further the color tone of reflected light has a bluish tinge.
According to the present invention, for dissolving the problems mentioned in the above, there is provided an optical reflector having a glass substrate, wherein upon at least one of main surfaces thereof is applied a film having a refractive index n1 at a wavelength of 550 nm; a film having a refractive index n2 at said wavelength; a film having a refractive index n3 at said wavelength; and a film having a refractive index n4 at said wavelength, sequentially, wherein:
there is established a relationship as indicated below among the refractive indices n1, n2, n3 and n4;
n1xe2x89xa7n4 greater than n3 greater than n2
film thickness of said film of the refractive index n2 is equal to or greater than 30 nm and is equal to or less than 60 nm;
a reflectivity to visible light of said film of the refractive index n4 is equal to or greater than 75%; and
a value of {(a*)2+(b*)2}xc2xd is equal to or greater than 0 and is equal to or less than 10, when representing a reflective color tone upon the surface of said film of refractive indices n4 by a* and b* (psychometric chroma coordinates).
Also, according to the present invention, it is preferable that said film having the refractive index n2 is one which contains silicon oxide (SiO2) as a main ingredient thereof, that said film having the refractive index n3 is one which contains tin oxide (SnO) as a main ingredient thereof, said film having the refractive index n4 is one which contains titanium oxide (TiO2) as a main ingredient thereof, and said film having the refractive index n1 is one which contains silicon (Si) as a main ingredient thereof.
Further, according to the present invention, it is preferable that an undercoating film is formed between said film having the refractive index n1 and said glass substrate, and it is further preferable that said undercoating film is a film of tin oxide.
Also, according to the present invention, it is preferable that film thickness is equal to or greater than 15 nm and is equal to or less than 45 nm for the film having the refractive index n1; is equal to or greater than 10 nm and is equal to or less than 80 nm for the film having the refractive index n3; and is equal to or greater than 10 nm and is equal to or less than 80 nm for the film having the refractive index n4, and it may be possible that an opaque layer is formed on a side of said glass substrate, being reverse to that on which said films are formed.
Also, according to the present invention, there is provided a method for manufacturing the optical reflector having a glass substrate, comprising steps of: forming a film having a refractive index n1 at a wavelength of 550 nm; forming a film having a refractive index n2 at said wavelength; forming a film having a refractive index n3 at said wavelength; and forming a film having a refractive index n4 at said wavelength, sequentially, upon at least one of main surfaces of said substrate. The relative values of the refractive indices are represented as n1xe2x89xa7n4 greater than n3 greater than n2.
Further, according to the present invention, it is also preferable that an undercoating film is formed between said film having the refractive index n1 and said glass substrate, and that said undercoating film, said film having the refractive index n1, said film having the refractive index n2, said film having the refractive index n3 and said film having the refractive index n4 are formed, sequentially, by means of a pyrolytic method upon a surface of said glass substrate at high temperature.
Namely, according to the present invention, forming the film having the refractive index n1 at the wavelength of 550 nm; the film having the refractive index n2 at the said wavelength; the film having the refractive index n3 at the said wavelength; and the film having the refractive index n4 at the said wavelength, sequentially, upon at least one of main surfaces of the glass substrate, and making the film thickness of the film of the refractive index n2 equal or greater than 30 nm and is equal or less than 60 nm, it is possible to improve the efficacy of the optical reflector by employing film formation using a pyrolytic method, thereby providing the optical reflector of high performance and good durability, having a value of {(a*)2+(b*)2}xc2xd being equal to or greater than 0 and being equal to or less than 10, when representing a reflective color tone upon surface of said film of refractive indices n4 by a* and b* (psychometric chroma coordinates).